[v2 PATCH 2/9] mm: memcontrol: use shrinker_rwsem to protect shrinker_maps allocation

[Yang Shi <shy828301@xxxxxxxxx>]

Since memcg_shrinker_map_size just can be changd under holding shrinker_rwsem
exclusively, the read side can be protected by holding read lock, so it sounds
superfluous to have a dedicated mutex.  This should not exacerbate the contention
to shrinker_rwsem since just one read side critical section is added.

Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
---
 mm/internal.h   |  1 +
 mm/memcontrol.c | 17 +++++++----------
 mm/vmscan.c     |  2 +-
 3 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/mm/internal.h b/mm/internal.h
index c43ccdddb0f6..10c79d199aaa 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -108,6 +108,7 @@ extern unsigned long highest_memmap_pfn;
 /*
  * in mm/vmscan.c:
  */
+extern struct rw_semaphore shrinker_rwsem;
 extern int isolate_lru_page(struct page *page);
 extern void putback_lru_page(struct page *page);
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 29459a6ce1c7..ed942734235f 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -394,8 +394,8 @@ DEFINE_STATIC_KEY_FALSE(memcg_kmem_enabled_key);
 EXPORT_SYMBOL(memcg_kmem_enabled_key);
 #endif
 
+/* It is only can be changed with holding shrinker_rwsem exclusively */
 static int memcg_shrinker_map_size;
-static DEFINE_MUTEX(memcg_shrinker_map_mutex);
 
 static void memcg_free_shrinker_map_rcu(struct rcu_head *head)
 {
@@ -408,8 +408,6 @@ static int memcg_expand_one_shrinker_map(struct mem_cgroup *memcg,
 	struct memcg_shrinker_map *new, *old;
 	int nid;
 
-	lockdep_assert_held(&memcg_shrinker_map_mutex);
-
 	for_each_node(nid) {
 		old = rcu_dereference_protected(
 			mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true);
@@ -458,7 +456,7 @@ static int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
 	if (mem_cgroup_is_root(memcg))
 		return 0;
 
-	mutex_lock(&memcg_shrinker_map_mutex);
+	down_read(&shrinker_rwsem);
 	size = memcg_shrinker_map_size;
 	for_each_node(nid) {
 		map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
@@ -469,7 +467,7 @@ static int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
 		}
 		rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map);
 	}
-	mutex_unlock(&memcg_shrinker_map_mutex);
+	up_read(&shrinker_rwsem);
 
 	return ret;
 }
@@ -484,9 +482,8 @@ int memcg_expand_shrinker_maps(int new_id)
 	if (size <= old_size)
 		return 0;
 
-	mutex_lock(&memcg_shrinker_map_mutex);
 	if (!root_mem_cgroup)
-		goto unlock;
+		goto out;
 
 	for_each_mem_cgroup(memcg) {
 		if (mem_cgroup_is_root(memcg))
@@ -494,13 +491,13 @@ int memcg_expand_shrinker_maps(int new_id)
 		ret = memcg_expand_one_shrinker_map(memcg, size, old_size);
 		if (ret) {
 			mem_cgroup_iter_break(NULL, memcg);
-			goto unlock;
+			goto out;
 		}
 	}
-unlock:
+out:
 	if (!ret)
 		memcg_shrinker_map_size = size;
-	mutex_unlock(&memcg_shrinker_map_mutex);
+
 	return ret;
 }
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 48c06c48b97e..912c044301dd 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -184,7 +184,7 @@ static void set_task_reclaim_state(struct task_struct *task,
 }
 
 static LIST_HEAD(shrinker_list);
-static DECLARE_RWSEM(shrinker_rwsem);
+DECLARE_RWSEM(shrinker_rwsem);
 
 #ifdef CONFIG_MEMCG
 /*
-- 
2.26.2